Stent with reinforced struts and bimodal deployment

ABSTRACT

The invention is directed to an expandable stent for implantation in a body lumen, such as a coronary artery or peripheral vein. The stent consists of a plurality of radially expandable cylindrical elements generally aligned on a common longitudinal stent axis and interconnected by one or more interconnecting members placed so as to limit longitudinal contraction during radial expansion. The individual radially expandable cylindrical elements are formed in a serpentine pattern having bends alternating in peaks and valleys designed to expand evenly under radial stress, and to maximize the overall radial expansion ratio. Each peak and valley includes reinforcing members that extend across and proximate to each bend. Sizing and construction of the struts forming the peaks and valleys can create bimodal deployment wherein the struts bend under increasing stresses to enable the stent to expand to larger diameters.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to expandable endoprosthesisdevices, generally called stents, which are adapted to be implanted intoa body lumen of a patient, such as a blood vessel, to maintain thepatency thereof. These devices are useful in the treatment and repair ofatherosclerotic stenoses in blood vessels.

[0002] Stents are generally cylindrically-shaped devices which functionto hold open and sometimes to expand a segment of a blood vessel orother anatomical lumen. They are particularly suitable for use tosupport and hold back a dissected arterial lining which, if not sosupported and held, can occlude the fluid passageway therethrough.

[0003] A variety of devices are known in the art for use as stents andhave included: coiled wires in an array of patterns that are expandedafter having been placed intraluminally via a balloon catheter;helically-wound coiled springs manufactured from an expandable heatsensitive metal; and self-expanding stents inserted in a compressedstate and shaped in a zig-zag pattern. Some more examples are shown inU.S. Pat. No. 4,776,337 to Palmaz; U.S. Pat. No. 4,655,771 to Wallsten;U.S. Pat. No. 4,800,882 to Gianturco; U.S. Pat. No. 4,913,141 toHillstead; and U.S. Pat. No. 5,292,331 to Boneau.

[0004] Such prior art devices include an expandable intraluminalvascular graft that is expanded within a blood vessel by balloonassociated with, typically, a dilatation catheter. The graft may be awire mesh tube, a stainless steel tube with rectangular openings, or atube with honeycomb style openings. Another prior art device includes aprosthesis for transluminal implantation comprising a flexible tubularbody made of flexible thread elements wound together, each thread havinga helix configuration.

[0005] There are still more conventional endovascular stents. In onedesign, the wire stent has a generally cylindrical shape, wherein theshape is formed with alternating bent wire loops. Another conventionalstent design comprises a series of continuous corrugations compressedtogether to form a tube-like mesh. Yet another endovascular stent usedfor the treatment of restenosis is a unitary wire structure, shaped tocriss-cross and form a plurality of upper and lower peaks.

[0006] One of the difficulties encountered using prior art stentsinvolved maintaining the radial rigidity needed to hold open a bodylumen while at the same time maintaining the longitudinal flexibility ofthe stent to facilitate its delivery. Another problem area was thelimiting range of expandability. Certain prior art stents expanded onlyto a limited degree due to the uneven stresses created upon the stentsduring radial expansion. This necessitated providing stents having avariety of diameters, thus increasing the cost of manufacture.Additionally, having a stent with a wider range of expandability allowedthe physician to re-dilate the stent if the original vessel size wasmiscalculated.

[0007] Another problem with the prior art stents was that the stentcontracted along its longitudinal axis upon radial expansion of thestent. This caused placement problems within the artery duringexpansion.

[0008] Various means have been devised to deliver and implant stents.One method frequently described for delivering a stent to a desiredintraluminal location involved mounting the expandable stent on anexpandable member, such as an inflatable balloon. The balloon wasprovided on the distal end of an intravascular catheter. The catheterwas advanced to the desired location within the patient's body lumen.Inflating the balloon on the catheter deformed the stent to apermanently expanded condition. The balloon was then deflated and thecatheter removed.

[0009] What has been needed and heretofore unavailable is a stent whichhas a high degree of flexibility so that it can be advanced throughtortuous passageways and can be radially expanded over a wide range ofdiameters with minimal longitudinal contraction, and yet have themechanical strength to hold open the body lumen into which it isexpanded. There is further a need for a stent-that has highcircumferential or hoop strength to improve crush resistance. Thepresent invention satisfies these needs.

SUMMARY OF THE INVENTION

[0010] The present invention is directed to an expandable stent having aconfiguration generally of the type disclosed in U.S. Pat. Nos.5,569,295 to S. Lam and 5,514,154 to Lau et al., the entire contents ofwhich are incorporated herein by reference. In a preferred embodiment,the present invention stent includes a plurality of adjacent cylindricalelements which are expandable in the radial direction and which arearranged in alignment along a longitudinal stent axis. The cylindricalelements are formed in a serpentine wave pattern transverse to thelongitudinal axis and contain a plurality of alternating peaks andvalleys.

[0011] The present invention also comprises at least one interconnectingmember that extends between adjacent cylindrical elements and connectsadjacent cylindrical elements to each other. The interconnecting membersinsure minimal longitudinal contraction of the stent during radialexpansion of the cylindrical elements.

[0012] The present invention further comprises, in each cylindricalelement, a reinforcing member that extends across each peak and valley.More precisely, each peak and each valley of a single cylindricalelement is formed by the confluence of two straight struts joining at abend. The reinforcing member thus spans across the peak or valley,bridging the struts.

[0013] The reinforcing member lends strength to the alternating peaksand valleys, wherein the area of maximum stress is at or near the bend.To be sure, the reinforcing member prevents the straight section of thestrut from buckling or distorting during expansion of the stent byadding material to a potentially weak area. Furthermore, the size andgeometry of the reinforcing member along with the bend may be adjustedso that stress is evenly distributed between the two instead of justbeing carried by the bend.

[0014] Certainly the geometry of the reinforcing member in the presentinvention can assume many configurations. For example, the reinforcingmember could include a loop that curves toward or away from the bend.The reinforcing member could join the struts at a point farther awayfrom or closer to the bend. The reinforcing member can be formed intothe bend.

[0015] The resulting stent structure is preferably a series ofradially-expandable cylindrical elements that are spaced longitudinallyclose enough to each other so that small dissections in the wall of abody lumen may be pressed back into position by the elements against thelumenal wall, but not so close as to compromise the longitudinalflexibility of the stent. The individual cylindrical elements may rotateslightly relative to adjacent cylindrical elements without significantdeformation, cumulatively providing a stent which is flexible along itslength and about its longitudinal axis, but which is still very stablein the radial direction in order to resist collapse.

[0016] The stent embodying features of the present invention can bereadily delivered to the desired lumenal location by mounting it on anexpandable member of a delivery catheter, for example a balloon, and bythen passing the catheter-stent assembly through the body lumen to theimplantation site. A variety of means for securing the stent to theexpandable member on the catheter for delivery to the desired locationare available. It is presently preferred to compress the stent onto theballoon. Other means to secure the stent to the balloon includeproviding ridges or collars on the inflatable member to restrain lateralmovement, or using temporary, bioabsorbable adhesives.

[0017] The present invention by use of the reinforcing members featuresbimodal deployment. That is, when the stent is expanded radially asdescribed above, it does so in two stages. The first stage is the typeof expansion of the stent radially wherein the struts bend slightlyoutward to accommodate the increasing circumference of each cylindricalelement and the loop portion of the reinforcing member is stretched out.The second stage continues from the first stage with the strutscontinuing to bend outward and with the most severe bending occurring atthe reinforcing member until the struts are pulled wide apart to theirlimits to accommodate the largest diameter that the stent can assume.Further spreading apart of the struts is prevented by the presence ofthe reinforcing member, which limits the maximum circumferential sizeattainable by each cylindrical element. By choosing the size andgeometry of the reinforcing member and the struts, the amount of forceneeded to expand the stent to a particular diameter can be altered.

[0018] The cylindrical elements of the stent are preferably plasticallydeformed when expanded (except when nickel-titanium (NiTi) alloys areused as the elements) so that the stent remains in the expandedcondition. Therefore, when non-NiTi elements are used, the elements mustbe sufficiently rigid when expanded to prevent the collapse thereof inuse. With super-elastic NiTi alloys, the expansion occurs when thestress of compression is removed which relief causes the phasetransformation of the material from the martensite phase back to theexpanded austenite phase.

[0019] After the stent is expanded, some of the peaks and/or valleys maytip outwardly and become embedded in the vessel wall. Thus, afterexpansion, the stent does not have a smooth outer wall surface, butrather is characterized by projections which embed in the vessel walland aid in retaining the stent in place in the vessel.

[0020] Other features and advantages of the present invention willbecome more apparent from the following detailed description of theinvention, when taken in conjunction with the accompanying exemplarydrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is an elevational view, partially in section, depicting astent embodying features of the invention which is mounted on a deliverycatheter and disposed within a body lumen such as a coronary artery.

[0022]FIG. 2 is an elevational view, partially in section, similar tothat shown in FIG. 1, wherein the stent is expanded within the artery,pressing the dissected lining against the arterial wall.

[0023]FIG. 3 is an elevational view, partially in section, showing theexpanded stent within the vessel after withdrawal of the deliverycatheter.

[0024]FIG. 4 is an enlarged partial view of the stent of FIG. 5depicting a serpentine pattern having peaks and valleys that form thecylindrical elements of the stent.

[0025]FIG. 5 is a plan view of a flattened section of a stent of thepresent invention which illustrates the serpentine pattern of the stent.

[0026]FIG. 6 is a side elevational view of the stent in the expandedcondition.

[0027] FIGS. 7(A)-(L) are top plan views of alternative embodiments of asingle reinforced peak or valley.

[0028]FIG. 8 is a plan view of an alternative embodiment of the presentinvention reinforced stent.

[0029]FIG. 9 is another alternative embodiment of the present inventionreinforced stent.

[0030] FIGS. 10(A) and (B) show the bimodal deployment of a preferredembodiment stent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031]FIG. 1 illustrates stent 10, incorporating features of theinvention, which is mounted onto delivery catheter 11. The stent 10generally comprises a plurality of radially expandable cylindricalelements 12 disposed coaxially and interconnected by members 13 disposedbetween adjacent cylindrical elements. The delivery catheter 11 has anexpandable portion or balloon 14 for expanding stent 10 within an artery15 or other vessel. The artery 15, as shown in FIG. 1, has a dissectedlining 16 which has occluded a portion of the arterial passageway.

[0032] The delivery catheter 11 onto which stent 10 is mounted isessentially the same as a conventional balloon dilatation catheter forangioplasty procedures such as percutaneous transluminal angioplasty(PTA) or percutaneous transluminal coronary angioplasty (PTCA). Theballoon 14 may be formed of suitable materials such as polyethylene,polyethylene terephthalate, polyvinyl chloride, nylon and ionomers suchas those manufactured under the trademark SURLYN by the Polymer ProductsDivision of the Du Pont Company. Other polymers also may be used. Inorder for stent 10 to remain in place on balloon 14 during delivery tothe site of the damage within artery 15, stent 10 is compressed onto theballoon. An elastic protective sheath is sometimes attached aroundballoon 14 so that stent 10 is crimped onto the sheath, which protectsthe balloon from the metal stent 10 and insures uniform expansion of thestent when the balloon and elastic sheath are expanded. A retractableprotective delivery sleeve 20 also may be provided to further ensurethat the stent stays in place on the expandable portion of deliverycatheter 11 and to prevent abrasion of the body lumen by the opensurface of stent 10 during delivery to the desired arterial location.Other means for securing stent 10 onto balloon 14 also may be used, suchas providing collars or ridges on the ends of the working portion, i.e.,the cylindrical portion, of the balloon.

[0033] Each radially expandable cylindrical element 12 of stent 10 maybe independently expanded. Therefore, balloon 14 may be provided with aninflated shape other than cylindrical, e.g., tapered, to facilitateimplantation of the stent 10 in a variety of body lumen shapes.

[0034] In a preferred embodiment, the delivery of the stent 10 isaccomplished in the following manner. The stent 10 is first mounted ontoinflatable balloon 14 on the distal extremity of delivery catheter 11.The stent may be “crimped” down onto the balloon to ensure a lowprofile. The catheter-stent assembly can be introduced within thepatient's vasculature in a conventional Seldinger technique through aguiding catheter (not shown). A guidewire 18 is disposed across thearterial section with the detached or dissected lining 16 and then thecatheter-stent assembly is advanced over guidewire 18 within artery 15until stent 10 is positioned within the artery at detached lining 16.The balloon 14 of the catheter is expanded, expanding stent 10 againstartery 15, which is illustrated in FIG. 2. While not shown in thedrawing, artery 15 preferably can be expanded slightly by the expansionof stent 10 to seat or otherwise fix stent 10 to prevent movement withinthe artery. In some circumstances during the treatment of stenoticportions of an artery, the artery may have to be expanded considerablyin order to facilitate passage of blood or other fluid therethrough.

[0035] Stent 10 serves to hold open artery 15 after catheter 11 iswithdrawn, as illustrated by FIG. 3. Due to the formation of stent 10from an elongated tubular member, the undulating component of thecylindrical elements of stent 10 is relatively flat in transversecross-section, so that when the stent is expanded, the cylindricalelements are pressed into the wall of artery 15 and as a result minimizethe development of thrombosis in artery 15. The cylindrical elements 12of stent 10 which are pressed into the wall of artery 15 eventually willbe covered with endothelial cell growth which further minimizesthrombosis. The serpentine pattern of cylindrical sections 12 providegood tacking characteristics to prevent stent movement within theartery. Furthermore, the closely spaced cylindrical elements 12 atregular intervals provide uniform support for the wall of artery 15, andconsequently are well adapted to tack up and hold in place small flapsor dissections in the wall of artery 15 as illustrated in FIGS. 2 and 3.

[0036] In the preferred embodiment, as depicted in FIGS. 4, 5 and 6, thestresses involved during expansion from a low profile to an expandedprofile are much more evenly distributed among the various peaks 36 andvalleys 34. As seen in FIG. 4, a portion of cylindrical element 12 ofstent 10 illustrates the serpentine pattern having a plurality of peaksand valleys, each having varying radii of curvature, which aids in theeven distribution of expansion forces. Interconnecting members 13 serveto connect adjacent valleys of cylindrical element 12 as describedabove.

[0037] After expansion, portions of the various elements will turnoutwardly, forming small projections which will embed in the vesselwall. For example, the tip of peak portion 36 tips outwardly uponexpansion a sufficient amount to embed into the vessel wall and helpsecure the implanted stent. Upon expansion, projecting peak 36 providesan outer wall surface on the stent that is not smooth, but instead has aplurality of projecting peaks 36 all along the outer wall surface. Whilethe projections assist in securing the stent in the vessel wall, theyare not sharp and thus do not cause trauma or damage to the vessel wall.

[0038] One important feature of the present invention is the capabilityof the stent to expand from a low-profile diameter to a diameter muchgreater than heretofore was available, while still maintainingstructural integrity of the stent in the expanded state. Due to itsnovel structure, the stent of the present invention has an overallexpansion ratio of 1 up to about 4 using certain compositions ofstainless steel. For example, a 316L stainless steel stent of thepresent invention can be radially expanded from a diameter of 1 unit upto a diameter of about 4 units, which deforms the structural membersbeyond their elastic limits. The stent still retains its structuralintegrity in the expanded state and it serves to hold open the vessel inwhich it is implanted. Materials other than 316L stainless steel maygive higher or lower expansion ratios without sacrificing structuralintegrity.

[0039]FIGS. 8 and 9 are plan views of a flattened section of stents 40,42 of the present invention, which illustrate the serpentine patterns ofthe stents as well as varying configurations of reinforcing members 44,46. In the preferred embodiment illustrated in FIG. 8, stent 40 iscomprised of a plurality of radially expandable cylindrical elements 48disposed generally coaxially and interconnected by interconnectingmembers 50.

[0040] As in the earlier described embodiments, the present preferredembodiment shown in FIG. 8 includes alternating peak portions 52 andvalley portions 54. Each peak portion 52 or valley portion 54 isessentially a bend 56 interconnecting straight struts 58. In thisembodiment, each peak portion 52 or valley portion 54 is reinforced byreinforcing member 44 extending across bend 56 to interconnect struts58. In the preferred embodiment depicted in FIG. 8, reinforcing member44 has an inverted loop 60 that extends in a direction opposite to bend56. Optionally, interconnecting members 50 may be integrated into loop60 of reinforcing member 44 as seen in FIG. 9.

[0041] The area of peak stress is at or near the apex of bend 56. Thepresent invention provides apparatus for reinforcing this area withreinforcing member 44, which is attached to each side of the bend (i.e.,strut 58) away from the apex of bend 56. The width of strut 58 alongwith the width and geometry of reinforcing member 44 as well as thegeometry and dimensions of bend 56 forming peak portion 52 or valleyportion 54 can be adjusted to distribute the stress between bend 56 andreinforcing member 44. Furthermore, varying the base material of thestent would affect the design of bend 56 and reinforcing member 44.

[0042] In FIG. 7, a variety of alternative embodiments of a peak portionor valley portion of a stent are shown. Specifically, reinforcingmembers of different constructions are shown in plan views. As seen inFIG. 7(A), peak portion or valley portion 62 is formed by a bend 64supported by struts 66. Reinforcing member 68 has a V shape and isintegrated into bend 64. FIGS. 7(B) and (C) show varying bendthicknesses. FIG. 7(D) illustrates a reinforcing member 70 thatintersects struts 72 wherein the point of intersection creates sharpenedcorners 74 that are rounded in FIGS. 7(B), (C), (E), and (F). In FIGS.7(E) and (F), reinforcing member 70 has been moved farther down struts72 away from bend 64. FIG. 7(G) depicts an alternative embodimentwherein reinforcing member 76 has been integrated into bend 78. In FIG.7(H), reinforcing member 80 includes loop 82 that has been pinchedtogether. FIG. 7(I) is a plan view of an alternative embodimentreinforcing member 84 that has been integrated into bend 86 althoughslits 88 have been formed in the base material. In FIGS. 7(J), (K) and(L), the shape of open areas 90, 92 have been adjusted to vary thestrength at different parts of the stent. Moreover, in FIGS. 7(J), (K)and (L), reinforcing member 94 has its orientation reversed as comparedto the reinforcing members in the previous embodiments.

[0043]FIG. 9 is a plan view of an alternative embodiment stent 42wherein the pattern of peaks and valleys have been modified to providemultiple side-by-side valley portions 96. Furthermore, interconnectingmember 98 is attached to bend 100 and transitions into a strut 102 at anopposite end.

[0044] The present invention further includes a bimodal feature asillustrated in FIGS. 10(A) and (B). FIG. 10(A) shows a singlecylindrical element 104 having alternating peaks and valleys, whereineach peak and valley is formed by bend 106 joining two struts 108. Inthe conditions shown in FIG. 10(A), struts 108 have been slightly bent,which is the result of a first stage expansion of the stent therebyincreasing the circumference of the stent. Thus, struts 108 are nolonger parallel and have spread outwards. Reinforcing member 110 helpsmaintain the angle formed by struts 108.

[0045] Also, FIG. 10(A) shows the first mode in which reinforcing member110 straightens and locks into position; the loop or kink previouslyformed in reinforcing member 110 is straightened. Reinforcing member 110in this configuration provides substantial strength and stiffness to thestent.

[0046] In FIG. 10(B), the stent has been expanded to a second stagethereby increasing the circumference of the stent to a greater degreethan that shown in FIG. 10(A). As the stent is expanded further, struts108 bend at the intersections with reinforcing members 110 until theyare aligned with the circumference of the stent as shown in FIG. 10(B).At this point, the stent is fully deployed to its maximum diameter.Accordingly, struts 108 have been pulled straight and are nearlyparalleled with reinforcing member 110. In this mode, the stent hasreached its maximum circumference; further increases in the stent canconceivably be achieved by deformation in struts 108 and reinforcingmember 110. Essentially, the circumference of the stent can be increasedby stretching struts 108 and reinforcing members 110 further.

[0047] It is possible to deploy the stent and reinforcing member with orwithout two distinct modes. This behavior is controlled by the forcerequired to bend struts 108 at their intersection with reinforcingmember 110 as compared to the force required to bend and open the loopin the reinforcing member 110. The behavior can be controlled by therelative widths and lengths of the various structures.

[0048] The tubing may be made of suitable biocompatible material such asstainless steel, titanium, tantalum, super-elastic nickel-titanium(NiTi) alloys and even high strength thermoplastic polymers. The stentdiameter is very small, so the tubing from which it is made mustnecessarily also have a small diameter. For PCTA applications, and as anexample only, typically the stent has an outer diameter on the order ofabout 0.065 inches (0.165 cm) in the unexpanded condition, the sameouter diameter of the tubing from which it is made, and can be expandedto an outer diameter of about 0.200 inches (0.508 cm) or more. The wallthickness of the tubing is about 0.003 inches (0.008 cm). For stentsimplanted in other body lumens, such as in non-coronary PTAapplications, the dimensions of the tubing forming the stent arecorrespondingly larger. The dimensions of the stent will vary dependingupon the application and body lumen diameter in which the stent will beimplanted.

[0049] In the instance when the stent is made from plastic, it may haveto be heated within the arterial site where the stent is expanded tofacilitate the expansion of the stent. Once expanded, it would then becooled to retain its expanded state. The stent may be convenientlyheated by heating the fluid within the balloon or the balloon directlyby a known method. The stent may also be made of materials such assuper-elastic NiTi alloys. In this case the stent would be formed fullsize but deformed (e.g. compressed) into a smaller diameter onto theballoon of the delivery catheter to facilitate transfer to a desiredintraluminal site. The stress induced by the deformation transforms thestent from a austenite phase to martensite phase and upon release of theforce, when the stent reaches the desired intraluminal location, thestent expands due to the transformation back to the austenite phase.

[0050] While the invention has been illustrated and described herein interms of its use as an intravascular stent, it will be apparent to thoseskilled in the art that the stent can be used in other instances in allvessels in the body. Since the stent of the present invention has thenovel feature of expanding to very large diameters while retaining itsstructural integrity, it is particularly well suited for implantation inalmost any vessel where such devices are used. This feature, coupledwith limited longitudinal contraction of the stent when it is radiallyexpanded, provides a highly desirable support member for all vessels inthe body. Other modifications and improvements may be made withoutdeparting from the scope of the invention.

What is claimed is:
 1. A flexible stent for implantation in a body lumenand expandable from a contracted condition to an expanded condition,comprising: a plurality of adjacent cylindrical elements which areexpandable in the radial direction and arranged in alignment along alongitudinal stent axis; the cylindrical elements formed in a serpentinewave pattern transverse to the longitudinal axis and containing aplurality of alternating peaks and valleys; at least one interconnectingmember extending between adjacent cylindrical elements and connectingthem to one another; at least one reinforcing member extending across awidth of the alternating peaks and valleys; the serpentine patterncontaining varying degrees of curvature in regions of the peaks andvalleys adapted so that radial expansion of the adjacent cylindricalelements is substantially uniform around their circumferences duringexpansion of the stent from its contracted condition to its expandedcondition.
 2. The stent of claim 1 , wherein the stent further comprisesat least one reinforcing member extending across a width of each of thealternating peaks and valleys.
 3. The stent of claim 1 , wherein theinterconnecting member connects a valley of one cylindrical element witha valley of an adjacent cylindrical element.
 4. The stent of claim 1 ,wherein the interconnecting member connects a reinforcing member of avalley of one cylindrical element with a valley of an adjacentcylindrical element.
 5. (1) The stent of claim 3 , wherein theinterconnecting member is unitary with the valley of one cylindricalelement and the valley of the adjacent cylindrical element.
 6. The stentof claim 1 , wherein the reinforcing member is curved opposite to therespective peaks and valleys.
 7. The stent of claim 1 , wherein thealternating peaks and valleys are further comprised of straight-lengthstruts intersecting at an angle, and wherein the reinforcing memberengages the intersecting struts at bend points.
 8. The stent of claim 7, wherein each bend point is a portion of the strut having reducedmaterial to facilitate bending.
 9. The stent of claim 1 , wherein thealternating peaks and valleys are further comprised of elongatedstraight-length struts intersecting at an angle, and wherein thereinforcing member engages the intersecting struts at bend points of theelongated struts.
 10. The stent of claim 1 , wherein the reinforcingmember is comprised of a first quarter turn that transitions into a halfturn, which transitions into a second quarter turn.
 11. The stent ofclaim 1 , wherein an intersection of the reinforcing member and thepeaks and valleys is rounded.
 12. The stent of claim 1 , wherein anintersection of the reinforcing member and the peaks and valleys isangular.
 13. The stent of claim 1 , wherein the reinforcing member isfurther comprised of an enlarged area integrated into the peak andvalley.
 14. The stent of claim 1 , wherein the reinforcing member isfurther comprised of an enlarged area integrated into the peak andvalley having slits therethrough.
 15. The stent of claim 1 , whereinsaid stent is formed of a biocompatible material selected from the groupconsisting of stainless steel, tungsten, tantalum, super-elastic NiTialloys, and thermoplastic polymers.
 16. The stent of claim 1 , whereinthe stent is formed from a single piece of tubing.
 17. The stent ofclaim 1 , wherein the stent is coated with a biocompatible coating. 18.A longitudinally flexible stent for implanting in a body lumen andexpandable from a contracted condition to an expanded condition,comprising: a plurality of adjacent cylindrical elements which areindependently expandable in the radial direction and arranged inalignment along a longitudinal stent axis; the cylindrical elementsformed in a serpentine wave pattern transverse to the longitudinal axisand containing alternating peaks and valleys; at least oneinterconnecting member extending between adjacent cylindrical elementsand connecting them to one another; a reinforcing member extendingacross each peak and valley; and the serpentine wave pattern configuredin size and shape so that the cylindrical elements generally expand in auniform manner around their circumferences during expansion of the stentfrom its contracted condition to its expanded condition.
 19. The stentof claim 18 , wherein within a single cylindrical element, theserpentine wave pattern includes a sequence containing a peak, a valley,a peak, a valley, a valley, and a peak.
 20. The stent of claim 18 ,wherein said at least one interconnecting member connects a valley ofone cylindrical element with a valley of an adjacent cylindricalelement.
 21. The stent of claim 18 , wherein the stent is formed of abiocompatible material selected from the group consisting of stainlesssteel, tungsten, tantalum, super-elastic NiTi alloys, and thermoplasticpolymers.
 22. A method for constructing a flexible stent forimplantation in a body lumen wherein the stent is expandable from acontracted condition to an expanded condition comprising the steps of:providing a plurality of adjacent cylindrical elements which areindependently expandable in the radial direction and arranged inalignment along a longitudinal stent axis; forming the cylindricalelements in a serpentine wave pattern transverse to the longitudinalaxis and containing a plurality of alternating peaks and valleys;providing at least one interconnecting member extending between adjacentcylindrical elements and connecting them to one another; providing atleast one reinforcing member extending across a width of the alternatingpeaks and valleys; and wherein the irregular serpentine pattern containsvarying degrees of curvature in regions of the peaks and valleys adaptedso that radial expansion of the adjacent cylindrical elements issubstantially uniform around their circumferences during expansion ofthe stent from its contracted condition to its expanded condition. 23.The process of claim 18 , wherein the process further comprises the stepof connecting the interconnecting member between a valley of onecylindrical element with a valley of an adjacent cylindrical element.